1. Field of the Invention
The present invention relates to a thin film transistor. In particular, the present invention relates to a display device provided with a thin film transistor.
2. Description of the Related Art
In recent years, a thin film transistor (hereinafter referred to as a TFT) including a semiconductor thin film (with a thickness of several nanometers to several hundreds of nanometers, approximately) over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attention. TFTs are widely used for IC (integrated circuit) and electronic devices such as electro-optical devices. In particular, a TFT is rapidly developed as a switching element of image display devices typified by liquid crystal display devices and the like. In an image display device such as a liquid crystal display device, a TFT including an amorphous semiconductor film or a TFT including a polycrystalline semiconductor film is mainly used as a switching element.
A TFT including an amorphous semiconductor film has low mobility. That is, current drive capability is low. Thus, there is a problem in that, when a protection circuit is formed using a TFT including an amorphous semiconductor film, a large-sized transistor should be formed as a countermeasure against electrostatic breakdown, which leads to hindrance to narrower frame parts. Further, there is also another problem in that parasitic capacitance between a scan line electrically connected to a gate electrode and a signal line electrically connected to a source electrode or a drain electrode may be increased when a large-sized transistor is formed, and thus power consumption may be increased.
On the other hand, a TFT including a polycrystalline semiconductor film has higher mobility than a TFT including an amorphous semiconductor film by two or more digits, and a pixel portion of a liquid crystal display device and peripheral driver circuits thereof can be formed over one substrate. A process of a TFT including a polycrystalline semiconductor film, however, is more complicated than that of a TFT including an amorphous semiconductor film, because of crystallization of a semiconductor film, introduction of an impurity element (doping), and the like. Therefore, there is a problem of a low yield and high cost. As a method for forming a polycrystalline semiconductor film, for example, a technique has widely been know in which the shape of a pulsed excimer laser beam is processed linearly by an optical system and an amorphous semiconductor film is irradiated with a linear laser beam while being scanned by the linear laser beam to be crystallized.
Note that as well as a TFT including an amorphous semiconductor film or a TFT including a polycrystalline semiconductor film, a TFT including a microcrystalline semiconductor film is known as a switching element of an image display device (for example, see Patent Document 1). However, there is difficulty in obtaining sufficient on current with a TFT including a microcrystalline semiconductor film. Therefore, in the TFT including an amorphous semiconductor film or a microcrystalline semiconductor film, various developments have been conducted to obtain sufficient on current.